Genuine superhero

This kid has been born with a rare genetic disorder (myostatin-related muscle hypertrophy) that gives him 40% more muscle mass than normal and minimal bodyfat. Furthermore, there appear to be no negative side affects. The disorder only affects skeletal muscle and not cardiac muscle. The only potential downside appears to be a possibility of neurological development disorders due to insufficient bodyfat.

It would be a disorder in nature. It's only really useful in a modern, well-fed society. If a famine hit, he'd be dead very fast because his body couldn't catabolize his muscles (which both provides energy and reduces the amount of energy needed per day). Same goes for the minimal fat stores -- those are your body's emergency reserves. In the modern (well, modernized) world, lack of food isn't really much of a problem. But historically (and in not yet modernized places), famine was a big problem.

Actually, the kid is quite underweight for his age even living in a time/country/household that can feed his crazy metabolism. This is like assuming a genetic mutation that gave a kid unrestricted brain growth would necessarily make him the smartest man alive. It's possible, but it's also possible his massive brain could be damaged by the lack of oxygen being pumped in through his non-adapted circulatory system.

Its a disorder because its not normal and going to cause complications in his future. His growth will be stunted for one thing. He will have lots of muscle but be small in height. Also, the amount of food he needs is enormous. The early years where his body (and brain) are developing will be damaged if he is not fed enough, which probably means they will have to overfeed him. He just burns calories too fast and it will affect his brain growth.

Also, as stated before, the next stage in evolution would be enabling us to get by during famine, with less food, not by making us eat more and hindering growth.

Im not sure if this is the same kid but I saw thing awhile ago about a kid who lacked a certain type of non essential protein that caused similar side effects. Probably not the next step in evolution but hell I wouldnt mind doing the iron cross at 5 months.